Amines
Easy Overview
Amines are basically ammonia (NH3) with one or more hydrogens replaced by carbon groups. They smell like rotten fish (sorry) and are found in everything from amino acids to medicines. This chapter covers how basic they are, how to make them, and what happens when you react them with nitrous acid.
Classification and structure
Primary (1°) amines: R-NH2. Secondary (2°): R2-NH. Tertiary (3°): R3-N. Quaternary ammonium salts: R4-N+. The classification depends on how many carbons are attached to nitrogen. Each type behaves slightly differently. Aniline is a primary aromatic amine — the most famous one.
Basicity of amines
Amines are basic because the nitrogen has a lone pair that can grab H+. Aliphatic amines are more basic than ammonia. Why? Alkyl groups push electrons toward nitrogen, making the lone pair more available. Aromatic amines (like aniline) are less basic — the lone pair gets delocalized into the benzene ring. It's like the lone pair is distracted.
Preparation of amines
Two main ways. Reduction of nitro compounds: nitrobenzene → aniline (using Sn/HCl or H2/Pd). Or Gabriel phthalimide synthesis: phthalimide + KOH + R-X → primary amine. The Gabriel method gives only primary amines with no secondary/tertiary contamination — super clean.
Diazonium salts
Primary aromatic amines + NaNO2 + HCl at 0-5°C → diazonium salt (Ar-N2+ Cl-). This is HUGE. Diazonium salts are extremely versatile — you can replace the -N2+ group with -Cl, -Br, -I, -CN, -OH, -H, and more. It's like a molecular Lego adapter that lets you swap in almost any group.
Reactions of diazonium salts
Substitution reactions: Sandmeyer reaction (CuCl/KCl → chloro), Gatterman reaction (Cu/HCl). Coupling reaction: diazonium salt + phenol or aromatic amine → azo dye (brightly colored). Azo dyes are used in textiles, food coloring, and even some medicines. The azo group (-N=N-) gives intense color.
Key Points
- •1° amine: R-NH2; 2°: R2NH; 3°: R3N
- •Aliphatic amines are more basic than ammonia
- •Aniline is less basic due to resonance of lone pair into benzene ring
- •Gabriel phthalimide synthesis gives pure primary amine
- •Diazotization: Ar-NH2 + NaNO2 + HCl at 0-5°C → Ar-N2+ Cl-
- •Sandmeyer: diazonium → chloro/bromo/cyano using CuX or CuCN
- •Coupling: diazonium + phenol → azo dye (colored compound)
Practice Questions
- Explain why aniline is less basic than methylamine.
- Write the mechanism of diazotization and Sandmeyer reaction.
- How are amines classified? Give one example of each class.
- What is the carbylamine reaction? Which amines give this test?
- Describe the preparation of azo dyes and give their significance.